Terminal deoxynucleotidyl transferase (TdT) is uniquely localized in thymus and bone marrow lymphocytes. This project will examine the specific biochemical functions, embryological sources, and progeny biological functions of prelymphocytes that differentiate through the TdT[unreadable]+[unreadable] stage. These experiments involve rat, mouse, and chicken cells from embryonic tissue and lymphoid organs. Attempts will be made to stimulate TdT[unreadable]-[unreadable] cells from embryo sources to positivity and TdT[unreadable]+[unreadable] cells from postnatal animals to TdT[unreadable]-[unreadable] functioning lymphocytes. The biochemical consequences of the presence of TdT in cells will be examined at the DNA level. Since we have now cloned the human TdT-CDNA sequence and expressed it in E. Coli, direct experiements at the DNA level are now possible. The results should provide information about the role of terminal transferase in development of the immune system. (LB)